An organic light emitting diode is a light emitting element utilizing organic electro-luminescence (hereinafter, referred to as organic EL (Organic Electro-Luminescence)), and generally has a constitution that an organic EL layer including a light emitting layer, which contains an organic light emitting material, is sandwiched between an anode and a cathode.
It is to be noted that such an organic EL layer is constituted of an electron injection layer, an electron transport layer, a hole injection layer, a hole transport layer and the like as needed other than the light emitting layer.
Further, the organic light emitting diode is divided into a bottom emission-type and a top emission-type depending on a difference of a light extraction surface being a surface from which light is extracted to the outside from the light emitting layer.
In a bottom emission-type organic light emitting diode, an anode made of a transparent conductive material such as indium tin oxide (ITO), an organic EL layer including a light emitting layer, and a cathode made of a metal material are formed sequentially on a transparent substrate such as a glass substrate, and light is extracted from a substrate side. Specifically, a substrate surface is a light extraction surface in the bottom emission-type organic light emitting diode.
On the other hand, in a top emission-type organic light emitting diode, a cathode made of a metal material, an organic EL layer including a light emitting layer, and an anode made of a transparent conductive material such as ITO are formed sequentially on a transparent substrate such as a glass substrate, and light is extracted from a side opposite to the substrate side, that is, from an anode side. Specifically, an anode surface is a light extraction surface in the top emission-type organic light emitting diode.
Further, in such a top emission-type organic light emitting diode, a top emission-type organic light emitting diode having a constitution that the cathode is set on the top is known other than the top emission-type organic light emitting diode in which the anode is at the top (namely, anode is provided for a side from which light is extracted) as described above.
Specifically, in this top emission-type organic light emitting diode with the cathode at the top, a reflective layer made of a metal material, an anode made of a transparent conductive material such as ITO, an organic EL layer including a light emitting layer, and a cathode made of a metal material are formed sequentially on a substrate (not limited to a transparent substrate such as glass), and furthermore, a layer is formed on the cathode by a transparent conductive material such as ITO, and light is extracted from a side opposite to the substrate side, that is, from a cathode side. In other words, in the top emission-type organic light emitting diode with the cathode at the top, the cathode surface is a light extraction surface. For this reason, in the top emission-type organic light emitting diode with the cathode at the top, the metal layer of the cathode is formed thin to allow light to be transmissible.
More particularly, the top emission-type organic light emitting diode with the cathode at the top has a constitution that at least a reflective layer made of a metal material, an anode conductive layer made of a transparent conductive material, an organic EL layer including a light emitting layer which contains an organic light emitting material, and a cathode conductive layer in which a light-transmissible metal layer and a transparent conductive layer made of a transparent conductive material are sequentially stacked on the substrate.
Features of such an organic light emitting diode are advantages that less view angle dependency, less power consumption, it can be fabricated extremely thin, and, on the other hand, a problem that light extraction efficiency being efficiency of extracting light from the light emitting layer to the outside is low.
The light extraction efficiency means ratio of light quantity radiated from a light extraction surface (the substrate surface in the bottom emission-type, the anode surface in the top emission-type with the anode at the top, the cathode surface in the top emission-type with the cathode at the top) to the outside of the organic light emitting diode, that is, into a free space.
In the organic light emitting diode, light from the light emitting layer emits in all directions, and much of the light takes a waveguide mode that light repeats total reflection on the interface of a plurality of layers having different refraction indices, and the light is converted into heat as its wave is guided in the layers, or radiated from a side surface which is not adjacent to other layers in each layer, by which light extraction efficiency was reduced.
Further, since a distance between the light emitting layer and the cathode formed by metal is close, a part of near field light from the light emitting layer was converted into surface plasmon on the surface of the cathode and lost, by which light extraction efficiency was reduced.
Since light extraction efficiency in the organic light emitting diode affects brightness of a display or illumination device using the organic light emitting diode, method for improving efficiency fare being considered.
Conventionally, regarding the bottom emission-type monochromatic organic light emitting diode, there has been known a method for implementing a fine uneven structure made of a lattice having a single period (corresponding wavelength is a narrow bandwidth) into an element. The single lattice converts surface plasmon, which occurs on the cathode surface (light emitting layer side) in the bottom emission-type monochromatic organic light emitting diode element, into propagation light and extracts the light, thereby improving light extraction efficiency of the bottom emission-type monochromatic organic light emitting diode.
For example, Patent Application No. 2010-246653 shows a relationship between a pitch of a periodic fine uneven structure and an extraction wavelength, in which the pitch of the periodic fine uneven structure corresponding to the maximum wavelength of a light emitting material constituting the organic light emitting diode could be known. Further, also in the case of the anode top emission-type element, which has a structure that the bottom side and the top side of a layer constitution of the bottom emission-type element are inverted, a relationship between the pitch of the periodic fine uneven structure and the extraction wavelength, which is shown in International Publication No. 2012/060404 Pamphlet shown as Patent Document 5, is established.
However, the relationship between the pitch of the periodic fine uneven structure and the extraction wavelength has not been known regarding the cathode top emission-type element. Because the cathode top emission-type element has a feature that a semi-transmissive electrode made of an ultrathin metal material is used as a cathode, the relationship between the pitch of the periodic fine uneven structure and the extraction wavelength shown in conventional Patent Document 5 could not be directly applied.
Actually, since the cathode top emission-type element is used regarding an active matrix type display using the organic light emitting diode, the fact that the most suitable fine uneven structure for increasing light extraction efficiency had been unknown was significantly inconvenient.
Further, regarding light extraction of the organic light emitting diode, an important problem other than the one described above is known. Specifically, in the case of implementing the fine uneven structure made of a lattice having a single periodic structure into an element in order to improve the light extraction efficiency of a monochromatic organic light emitting diode, regardless of a type of the layer constitution of the organic light emitting diode, there was a problem that effect of extraction efficiency improvement had not obtained unless matching between the pitch of the fine uneven structure and the extraction wavelength had been accurate.
This problem applies to all of the bottom emission-type element, the anode top emission-type element, and the cathode top emission-type element.
Then, a device for eliminating the need of matching between the fine uneven structure and the extraction wavelength was considered by using a fine uneven structure for a white organic light emitting diode (corresponding wavelength is broadband) (described later) in the monochromatic organic light emitting diode element. However, there was a problem that the light extraction efficiency of the fine uneven structure for the white organic light emitting diode had been worse than the light extraction efficiency of the fine uneven structure for the monochromatic organic light emitting diode in the case where a light emission wavelength was limited to a certain narrow bandwidth.
The present invention has been created in consideration of the above-described problems, and it proposes a novel fine uneven structure for single color which combines the advantage of the fine uneven structure for single color (single period) and the advantage of the fine uneven structure for white color.
It is to be noted that Patent Documents 1 to 5 disclose methods using surface plasmon resonance as a technique for improving light extraction efficiency.
Specifically, in Patent Documents 1 to 5, a one-dimensional or two-dimensional periodic fine uneven structure is provided on the surface of a metal layer (cathode), by which the periodic fine uneven structure functions as a diffraction grating, and surface plasmon of the metal layer (cathode) surface is radiated. Thus, energy lost as surface plasmon is extracted as light, and light extraction efficiency improves.
Patent Document 4 out of the above-described Patent Documents discloses a method in which an organic light emitting diode substrate having a periodic lattice structure being a uneven structure, which is fabricated by a dry etching method that uses a two-dimensional crystalline body made of a particle single layer film as an etching mask, is fabricated, and an anode conductive layer, an organic EL layer, and a cathode conductive layer are sequentially stacked on the organic light emitting diode substrate.
In other words, as the periodic lattice structure is formed on the monochromatic organic light emitting diode substrate surface, a shape of the periodic lattice structure formed on the organic light emitting diode substrate surface is sequentially transferred onto each electrode layer and organic EL layer when stacking the layers, so that a periodic lattice structure having a shape in which the periodic lattice structure of the organic light emitting diode substrate surface is copied is formed on the surface of the light emitting layer side of the cathode conductive layer, by which light extraction efficiency can be improved.
Herein, in an organic light emitting diode provided with the organic light emitting diode substrate in which the periodic lattice structure as described above is formed, there is known that intensity, angle and a wavelength range of light emitted from the organic light emitting diode can be changed by changing parameters such as the pitch and height of an uneven structure of the periodic lattice structure.
Conventionally, the periodic lattice structure has been fabricated to bring a period of the uneven structure to a constant level aiming at efficiently extracting light having a specific single wavelength being a desired wavelength to obtain strong light.
This is because the more constant a period of unevenness being the fine uneven structure which is the periodic lattice structure, that is, spacing of unevenness, the higher the light extraction efficiency to a certain wavelength became.
For example, FIG. 1(a) shown as a first conventional art regarding the periodic fine uneven structure shows an example of the case where an organic light emitting diode substrate having a periodic lattice structure in which the period of an uneven structure is constant is fabricated by a dry etching method using a two-dimensional crystalline body made of a particle single layer film whose particle diameter is a constant particle diameter D as an etching mask, as shown in (a-1) of FIG. 1(a), and (a-2) of FIG. 1(a) shows power spectrum of height distribution in the organic light emitting diode substrate surface having such a periodic lattice structure in which the period of the uneven structure is constant.
Specifically, in the case where the periodic lattice structure of the organic light emitting diode substrate surface is formed to bring the period of the uneven structure to a constant level, a delta functional dot sequence arrayed at positions of the apexes of a regular hexagon is obtained as the power spectrum which is obtained by performing two-dimensional Fourier transform to the height distribution in the uneven structure.
Then, (a-3) of FIG. 1(a) shows a profile of the power spectrum of the height distribution in the periodic lattice structure where the period of the uneven structure becomes constant.
Herein, the power spectrum of the height distribution is obtained by: transforming the height distribution of the uneven structure into wave-vector space by performing two-dimensional Fourier transform; finding power spectrum intensities (squared absolute values of amplitude); and plotting them.
Further, the profile of the power spectrum is obtained by: integrating the power spectrum intensities on a circumference where wave numbers become constant in the power spectrum; and plotting them with wave numbers taken on the axis of abscissa.
Coordinates of each point in the power spectrum space corresponds to a wave vector K=(Kx,Ky). An absolute value K=|K|=(Kx2+Ky2)1/2 of the wave vector is called a wave number. Further, the wave number is equivalent to a product obtained by multiplying a space frequency by 2π.
Such a profile of the power spectrum of the height distribution has high intensity at a specific wave number and includes a sharp peak as shown in (a-3) of FIG. 1(a), and indicates that surface plasmon having the specific wave number is diffracted to be converted into light. Consequently, the profile shows that light extraction efficiency of light having the same frequency as the surface plasmon is high.
However on the other hand, the fine uneven structure having high light extraction efficiency has a narrow allowable range of a deviation of period, so there was a problem of risk that the extraction wavelength is deviated from a target light emission wavelength when the fine uneven structure only slightly changed.
Therefore, in the case of aiming at obtaining broadband light such as the case where the light extraction efficiency of white light needs to be increased, it has been pointed out obtaining desired light was difficult on the organic light emitting diode substrate having a two-dimensional lattice structure whose period of the uneven structure was constant.
It is to be noted that the case of aiming at obtaining broadband light is the case of fabricating a white organic light emitting diode where a wavelength of light to be extracted spans the entire visible light region (380 nm to 780 nm), the case of extracting light in a broader band, which is light that spans the entire visible light to near infrared region (380 nm to 2500 nm) for example.
As a technique for solving such a problem, there is proposed a technique of making the pitch, height or the like of an uneven structure formed on the organic light emitting diode substrate surface irregular.
Further, FIG. 1(b) shown as a second conventional art regarding the periodic fine uneven structure shows an example of the case where an organic light emitting diode substrate having an uneven structure with an irregular pitch is fabricated by a dry etching method in which a two-dimensional crystalline body made of a particle single layer film which is formed by mixing particles of a plurality of particle diameters (two types or more for example) is used as an etching mask as shown in (b-1) of FIG. 1(b), and (b-2) of FIG. 1(b) shows the power spectrum of the height distribution in the organic light emitting diode substrate surface having such an uneven structure with the irregular pitch.
Specifically, in the case where the uneven structure of the organic light emitting diode substrate surface is formed to make the pitch irregular, the power spectrum of the height distribution showing distribution in a circular region is obtained.
The profile of the power spectrum of the height distribution in the uneven structure with the irregular pitch, which was obtained by the organic light emitting diode having the uneven structure with the irregular pitch includes a certain degree of intensity in a broadband wave number region as shown in (b-3) of FIG. 1(b), and its absolute value was not high compared with the uneven structure whose period is constant.
However, an attempt of obtaining power spectrum intensity by simply increasing the height of the uneven structure is not preferable because it causes a new problem that surface plasmon tends to be localized and a short circuit in an element easily occurs.
In other words, according to the organic light emitting diode having the uneven structure with the irregular pitch, although an organic light emitting diode having extraction efficiency of electromagnetic wave in an arbitrary broadband wavelength region from the visible light to near infrared region can be realized and there is no possibility that the extraction wavelength is deviated from a target light emission wavelength when the fine uneven structure only slightly changed, it caused a new problem that the intensity of light to be extracted becomes weak.